A. Field of the Invention
The present invention relates to a heat treatment apparatus for heat-treating circular-plate like articles to be treated such as semiconductor wafers or substrates and also relates to a heat treatment boat.
B. Description of the Related Art
As one of the processes for manufacturing semiconductor wafers or substrates, heat treatment is used for forming oxidized films and diffusing dopants at a high temperature. Horizontal heat treatment furnaces were mainly used in the past, but vertical heat treatment furnaces have recently come to be mostly used, because the latter takes in little air in the atmosphere.
A heat treatment apparatus using a vertical heat treatment furnace, i.e., a vertical heat treatment apparatus employs a vertically elongated heat treatment boat (or a wafer boat) accommodating a lot of wafers vertically spaced from one another and loading them in and unloading them from the vertical heat treatment furnace. The heat treatment boat comprises a circular top plate, a circular bottom plate disposed thereunder so as to be opposed thereto and four support rods made of quartz or the like and provided between the top and bottom plates. Two of the four support rods are arranged at such positions that they support the right and left portions of the front end part of each wafer in view of the direction in which the wafer enters the heat treatment boat. The support rods are provided on a heat insulating tube made of a heat insulating material.
Each support is formed with horizontal grooves having a width slightly larger than the thickness of a wafer fitted in the corresponding groove so as to support the corresponding outer peripheral edge of wafer. The wafers are fitted in and taken out of horizontal grooves of the support rods by a transfer arm in a space defined between the two front support rods.
After a predetermined number of wafers have been mounted on the heat treatment boat, it is lifted by an elevator and introduced into the heat treatment furnace. Then, the wafers are loaded in the heat treating furnace, and predetermined heat treatment is performed.
In one of heat treatment processes, wafers are heated at such a high temperature as 1200.degree. C. for a long time in order to diffuse, to a predetermined depth, dopants (impurity ions) which are formed by ion injection. When the mother material of the wafers is silicon, its yield stress at this high temperature becomes such an extremely small value as about 1/560 of its yield stress at the normal temperature (room temperature).
The diameter of wafers has become larger from 6 inches to 8 inches, and wafers having a diameter of 12 inches are now being developed. When, however, the wafers having such a large diameter are heat-treated at a high temperature close to the melting point of the mother material of the wafers, surface defects called slip are produced at the portions supported by the support rods of the heat treatment boat. The slip is formed by minute faults which cannot be visually observed but can be found by a magnifying lens or under a microscope.
The causes of slip generation are:
(1) the internal stress due to the weight of a wafer itself and
(2) thermal distortional stress due to uneven distribution of temperature on the wafer surface.
With respect to cause (1), since the wafer is supported at its circular outer peripheral edge by heat treatment boat and thus partially supported, a large internal stress is produced by the weight of the wafer itself at the supported portions. It is considered that slip is produced when the internal stress exceeds a predetermined value.
With respect to cause (2), it is considered that slip is produced when thermal distortional stress produced by the difference of the temperature distribution between the central portion and the peripheral portions of the wafer as the temperature is raised exceeds a predetermined value.
The mechanism as to how is produced the internal stress due to the weight of the wafer, which stress is considered to be one of the causes of slip generation, will be explained more in detail in view of the structure of the conventional heat treatment boat. Each wafer is not only bent within a manufacturing tolerance but also due to uneven temperature distribution as the heating temperature is raised, and each groove of each support rod has a manufacturing error. The bending and the manufacturing errors are combined to reduce number of the supporting points of the wafer from four to three as shown in FIG. 10. When the wafer is supported at three points, the loads on the three points become unbalanced as seen from the load distribution on the wafer around the support rods shown by Xs, and a large stress exceeding the limit of slip generation is exerted on one of the supported points Xs.
A method of reducing the internal stress due to the weight of a wafer itself has been developed in which the supporting areas of the wafer are widened by using the four support rods of the heat treatment boat made arcuated along the circular peripheral edge of each wafer.
However, it takes a lot of time and cost to machine members into support rods having an arcuated cross section for ensuring a large supporting area. In addition, each wafer is not always supported evenly by the four support rods over their whole supporting areas and is likely to receive an excessive load at its local portion. Thus, it is little guaranteed that slip is effectively protected by this method.
When, therefore, wafers having a large diameter are heat treated particularly at a high temperature close to the melting point of the mother material of the wafer, there occurs a problem that slip is generated. This makes it difficult to increase the diameter of the wafer.